The present invention relates to a method for producing amorphous silica from silicon and silicon-containing materials by flame combustion of silicon-containing materials with air, oxygen-enriched air or oxygen.
Amorphous silica, having a small particle size and a high surface area, is today mainly produced by reacting volatile silicon compounds, particularly silicon tetrachloride with an oxygen-enriched gas in a flame. This product, which is called xe2x80x9cfumed silicaxe2x80x9d, has a number of applications, particularly as a filler in plastic and rubber. It is further known that similar amorphous silica products can be produces in plasma furnaces and electric furnaces where the reactants are quarts and carbon or silica and quarts. These products are, however, costly and have not found much use.
From EP-B-151490 and U.S. Pat. No. 4,755,368, there are known methods for flame combustion or silicon, where particulate silicon is fluidized in a gas, such as air or oxygen, and supplied to a reactor. The silicon is combusted in a flame to produce SiO2 particles.
From Norwegian patent No. 304366, there is known a method for the production of amorphous silica from particulate silicon-containing materials, where particulate silicon is supplied in fluidized form to a combustion reactor using an oxygen-containing fluidizing gas and where additional oxygen-containing gas is supplied to the combustion reactor whereby the particulate silicon is burned to SiO2 without the addition of further heat energy.
It is difficult to obtain an even and consistent process in the known methods for combustion of silicon powder to amorphous SiO2. It is believed that a main reason for this is that it is difficult to obtain an even and constant supply of Si-powder to the reactor when the silicon powder is supplied to the reactor fluidized in a gas. Further, as can be seen from Norwegian patent No. 304366, only a specific surface area of about 20 m2/g for the produced SiO2 is obtained, while for many applications, amorphous silica is needed having a much higher surface area of 80 m2/g or higher.
It is an object of the present invention to provide a method and an apparatus for the combustion of silicon-containing materials to amorphous SiO2, whereby silicon is supplied to the combustion reactor in a very even and stable way and whereby amorphous silica is obtained having a surprisingly high specific surface area.
The present invention thus relates to a method for the production of amorphous silica from particulate materials containing at least 50% elemental silicon, where the silicon-containing material is supplied to a combustion reactor which is heated by an oil or gas burner, and where amorphous silica is recovered after the outlet from the combustion reactor. The method is characterized in that at least a part of the particulate silicon-containing materials are supplied to the combustion reactor in the form of an aqueous slurry.
According to a preferred embodiment, the silicon is supplied to the combustion reactor in the form of an aqueous slurry containing between 20 and 80% by weight silicon-containing material, and preferably between 40 and 60% by weight silicon-containing material.
According to another preferred embodiment, a viscosity regulating agent, such as a mineral acid or organic dispersing agents, are added to the aqueous slurry containing silicon-containing material.
In order to modify the surface of the produced amorphous silica, surface modifying agents can be added to the aqueous slurry, such as, salts of aluminum, preferably aluminum sulphate.
According to yet another embodiment, the amorphous silica is quenched by the addition of water in the area of the outlet from the combustion reactor.
It has surprisingly been found that by supplying silicon in the form of an aqueous slurry, a steady state operation in the reactor is obtained at the same time as the produced amorphous silica has a surprisingly high specific surface area of more than 100 m2/g.
Any particulate material having a content of elemental silicon of at least 50% can be used as the silicon-containing material. Examples of such materials are metallurgical grade silicon in particulate form, high purity silicon in particulate form, atomized silicon, kerf from the sawing of silicon wafers for electronics and for solar cells, filter dust obtained from the crushing of silicon and residues from reactors for the production of organohalosilanes and inorganic silanes.
The present invention further relates to an apparatus for the production of amorphous silica from particulate materials containing at least 50% elemental silicon, the apparatus comprising an elongated reaction chamber having an oil or gas burner in one end, at least one opening arranged along the sidewall of the combustion reactor for the supply of silicon-containing material in the form of an aqueous slurry, an outlet opening for produced amorphous silica, and a means for collecting amorphous silica.
The combustion reactor preferably has an inlet opening for the supply of air, oxygen enriched air or oxygen.